1. Field of the Invention
This invention relates to a hydrostatic positive displacement machine, in particular an axial piston machine with variable displacement volume, which can be operated in a closed circuit and delivers hydraulic fluid into a working circuit. The displacement volume can be adjusted by means of a control device which comprises a variable displacement piston device which is provided with a first control pressure chamber and a second control pressure chamber. The variable displacement piston device is in an operational connection with a displacement volume control mechanism, in particular a swashplate, whereby the control pressure chambers of the variable displacement piston device can be connected by means of a control valve with a feed pressure source or can be relieved to a reservoir. The positive displacement machine and the feed pressure source are driven by a drive machine, such as an internal combustion engine. The positive displacement machine has a swivel-back device which, in the event of a decrease in the feed pressure from the feed pressure source, guarantees the actuation of the positive displacement machine toward a reduction of the displacement volume.
2. Description of Related Art
Hydrostatic positive displacement machines with variable displacement volume which are operated in a closed circuit are used as pumps, e.g., in traction drives of mobile industrial trucks. The displacement volume of the positive displacement machine thereby determines the speed of travel of the work machine.
A control device is provided to set the displacement volume of the positive displacement machine. The control device has a variable displacement piston device which is in an operative connection with a displacement volume control mechanism that determines the displacement volume, for example a swashplate with a variable tilt, of a displacement machine in the form of an axial piston machine utilizing a swashplate construction. For the actuation of the variable displacement piston device, there is a control valve, for example an actuating pressure control valve, which is in communication with a feed pressure source and a reservoir and controls the actuation of the control pressure chambers of the variable displacement piston device with an actuating pressure generated from the feed pressure of the feed pressure source. The control device, which is acted on by the actuating pressure in the control pressure chamber, then generates an actuating torque on the displacement volume control mechanism which, together with an intrinsic torque of the displacement volume control mechanism which is generated from the forces acting on the piston of the positive displacement machine, produces the desired adjustment of the displacement volume control mechanism. The control valve in the form of an actuating pressure control valve is generally a position-controlled pilot valve which is actuated by an actuator signal which specifies a value for the displacement volume and to which the current position of the displacement volume control mechanism is fed back. The control device of the positive displacement machine for the adjustment of the displacement volume is a position feedback control.
The positive displacement machine and the feed pressure source, which is generally formed by a feed pressure pump and generates a constant feed pressure, are driven jointly by a drive machine, such as an internal combustion engine. The feed pressure pump is generally a constant displacement pump with a fixed displacement volume so that the delivery of the feed pump is proportional to the speed of rotation of the drive machine.
At low drive speeds of rotation of the drive machine and, therefore, the accompanying low feed volume flow of the feed pressure pump, there can be a drop or decline of the feed pressure of the feed pressure pump. On account of the drop in the feed pressure, when the control valve is actuated, operating conditions can occur on account of the reduced forces in the control pressure chamber of the variable displacement piston device in which the adjustment of the displacement volume control mechanism with the position control circuit can no longer be guaranteed and the positive displacement machine no longer operates with the intended positive ratio of actuating torque moment and the intrinsic torque.
In the event of a reduction in the speed of rotation of the drive machine and, therefore, an overloading of the drive machine, it is no longer possible to guarantee a desired adjustment of the displacement volume control mechanism toward a reduction of the displacement volume. Under some conditions, the result can be an adjustment of the displacement volume control mechanism toward an increase of the displacement volume by the intrinsic torque, as a result of which the drive machine can stall.
When a positive displacement machine of this type is used as a pump in a hydrostatic traction drive of a mobile work machine, such as a wheel loader, this type of stalling of the drive machine in the form of an internal combustion engine can occur under operating conditions in which the shovel is driven into a pile of debris and the hydraulic working system is also loaded, for example to tip or raise the shovel. Under these conditions, there can be a reduction in the speed of rotation of the drive machine with a corresponding decline or drop of the feed pressure. As a result of which, an additional load can be placed on the drive machine which leads to the stalling of the drive machine because, on account of the reduced feed pressure and the intrinsic torque, the displacement volume control mechanism of the positive displacement machine can no longer be adjusted in the direction of a reduction of the displacement volume to reduce the load on the drive machine.
To guarantee the adjustment of the displacement volume control mechanism under all operating conditions in the direction of a reduction of the displacement volume, the known art describes providing a reverse swiveling device with corresponding springs on the variable displacement piston which, in the event of a decline or drop in the feed pressure, applies a correspondingly high spring pressure to apply pressure to the variable displacement piston device and, thus, the displacement volume control mechanism toward a neutral position with a displacement volume of zero and, thus, toward a reduction of the displacement volume. A positive displacement machine with a reverse swiveling device is known, for example, from DE 40 02 017 A1. However, on account of the high spring forces on the variable displacement piston device, a reverse swiveling device of this type entails a great deal of construction effort and expense because the return forces of the springs, even in normal operation of the positive displacement machine, must be overcome when the displacement volume control mechanism is swiveled out by the control pressure present in the control pressure chamber of the variable displacement piston device, which means that a correspondingly high actuating pressure level is required, with correspondingly large actuating pressure chambers on the variable displacement piston device.
An object of this invention is to provide a positive displacement machine of the general type described above but with which, in the event of a drop in the feed pressure, a reliable adjustment of the displacement volume control mechanism toward a reduction of the displacement volume can be achieved at small expense in terms of time, materials, cost, and effort.